Digital Circuits

Review – Getting the truth table The first step in designing a digital circuit usually is to get the truth table. That is, for every input combination, figure out what an output bit should be, and write them down in a table.

Review – From the truth table to circuits Any truth table can be translated into a circuits consisting of several and gates followed by one or gate. – It means that any function can be implemented in this way Call a row in the truth table in which the output is `1’ a ``true row’’ and the input combination in this row a ``true input combination’’ or just a ``true combination.’’ Each and gate corresponds to one ``true row’’ in the truth table. The and gate should output a `1’ if and only if the input combination is the same as this row. If all other cases, the output of this and gate is `0.’ – So, whenever the input combination is the same as one of the ``true combinations,’’ one of the and gates outputs ``1’’, so the output of the or gate is 1. – If the input combination is not the same as any of the ``true combinations,’’ none of the and gates will output a ``1’’, so the output of the or gate is 0.

4 Logic Functions Drawing circuits is … Usually we express logic functions using logic equations which are more succinct and carry the same information – The OR operator is written as +, as in A + B. – The AND operator is written as ·, as A · B. – The unary operator NOT is written as or A’. Remember: This is NOT the binary field. Here 0+0=0, 0+1=1+0=1, 1+1=1.

Logic functions For example, the sum in the one-bit full adder is From a logic function you can immediately know what the circuit looks like. Truth table == Circuits == Logic function, equivalent. So we are going to get familiar with getting the logic functions from the truth table

Problems Ex 1. Assume that X consists of 3 bits, x2 x1 x0. Write a logic function that is true if and only if X contains only one 0

EX 1 X2X1X0output

EX 1 X2X1X0output

Ex 1 Output = x2x1x0’ + x2x1’x0 + x2’x1x0

Ex 2 Assume that X consists of 3 bits, x2 x1 x0. Write a logic functions that is true if and only if X contains an even number of 0s.

EX 2 X2X1X0output

EX 2 X2X1X0output

Ex 2 Output = x2x1’x0’ + x2’x1’x0 + x2’x1x0’+ x2x1x0

Ex 3 Assume that X consists of 3 bits, x2 x1 x0. Write a logic functions that is true if and only if X when interpreted as an unsigned binary number is no less than 5.

Ex 3 X2X1X0output

Ex 3 X2X1X0output

Ex 3 Output = x2x1’x0 + x2x1x0’+ x2x1x0

In class exercises 1 Assume that X consists of 3 bits, x2 x1 x0. Write a logic functions that is true if and only if X when interpreted as an unsigned binary number is less than 4.

In class exercises 2 Implement a circuit with three inputs (X2, X1, X0), and one output O. O should be 1 only when X2, X1, X0 are representing an odd binary number.